1. Technical Field
The present application relates to systems for mounting engines in aircraft. In particular, the present application relates to systems for mounting engines in tilt rotor aircraft.
2. Description of Related Art
Tilt rotor aircraft are unique in that they have tilt rotor assemblies that operate between a helicopter mode in which the tilt rotor assemblies are rotated upward allowing the tilt rotor aircraft to take off, hover, fly, and land like a conventional helicopter; and an airplane mode, in which the tilt rotor assemblies are tilted forward allowing the tilt rotor aircraft to fly like a conventional fixed-wing propeller driven aircraft. The first tilt rotor aircraft were designed strictly for military or research purposes, but now plans are being made to manufacture civilian-type tilt rotor aircraft.
Aside from the vast differences between tilt rotor aircraft and conventional helicopters and propeller driven aircraft, there are significant differences between military-type tilt rotor aircraft and civilian-type tilt rotor aircraft. Often a design that is necessary in a military-type application is not commercially feasible in a civilian-type application. For example, certain factors, such as weight, passenger comfort, safety, and cost, must be carefully considered if a civilian-type tilt rotor aircraft program is to be successful. In addition, civilian-type tilt rotor aircraft will have to comply with a considerable amount of governmental regulation. Certain additional design considerations must be made to accommodate the civilian tilt rotor aircraft market.
Although tilt rotor aircraft provide many unique advantages, they also present many unique challenges. One such challenge involves the mounting of the engines to the transmissions and other components in the aircraft. Because the engines in tilt rotor aircraft are mounted to pylons and transmissions located at the ends of the wings in a manner that allows rotation relative to the wings, engine mounting techniques used in conventional helicopters and propeller driven aircraft are not adequate.
In tilt rotor aircraft, the engines and transmissions are mounted in nacelles that rotate relative to the wings of the aircraft. The rotors, transmissions, and engines are carried by pylon assemblies within the nacelles that rotate about spindles that are installed into the outboard ends of the wing members. The transmissions, also referred to as prop rotor gear boxes, are coupled to the forward ends of the pylon assemblies, and the engines are disposed beneath the pylon assemblies. The engines are coupled to the transmissions and pylons by engine mounting systems. The engine mounting systems perform several functions: they hold and support the engine in the aircraft; they counteract the torque applied to the transmission by the engine input shaft; and they play a critical role in determining the contribution of the engine to the overall dynamic response of the aircraft.
In early designs, the engine was attached by an engine mount directly to the prop rotor gear box so that the engine extended back from the prop rotor gear box in a cantilevered fashion and hung generally parallel to and below the pylon assembly. However, it was quickly determined that support of the aft end of the engine was necessary. This was done initially with a single rigid link connected to the pylon. In later designs, the engine was attached to the prop rotor gear box by a gimbal assembly and supported at the aft end by multiple rigid links. One of the links was later modified to include a tuning means which could be used to counteract primarily lateral loads and to tune the dynamic response of the engine.
Although these changes in the engine mounting system solved the problems associated with the engines being cantilevered from the prop rotor gear boxes, they introduced other problems. Most significantly, coupling the engine to both the prop rotor gear box and the pylon assembly created a mechanical loop through which torque from the main rotor could be induced into the engine at the prop rotor gear box and reacted near the aft end of the engine into the pylon. Thus, the introduction of multiple rigid links coupled near the aft end of the engine created torsional redundancy in the engine. In other words, these prior-art techniques unnecessarily allowed a portion of the rotor torque to be induced into the engine-pylon loop.
The foregoing represents great strides in the design and manufacture of tilt rotor aircraft; however, many challenges remain, particularly in the area of engine mounting systems.